Microorganisms that are currently being used to ferment sugars to biofuels such as ethanol usually cannot utilize complex polysaccharides such as cellulose and hemicellulose. As a result, a significant bottleneck occurs in the conversion of lignocellulosic materials to biofuels. The cellulose component of plant matter may be hydrolyzed to glucose (a 6-carbon sugar) by a cellulase system, which usually comprises three important enzymes: an endoglucanase, an exoglucanase, and a beta-glucosidase. These enzymes, however, do not target the hemicellulose component of the plant material.
Hemicellulose is a complex polysaccharide that has a xylose-linked backbone, with side chains of arabinose, glucuronyl, and acetyl groups. A structural model of a hemicellulose illustrates the xylose backbone residues joined together in beta-1,4-linkages (FIG. 1a). Several functional groups decorate the backbone, including esters of acetyl (Ac) groups, arabinose, glucuronic acids, and esters of feroryl group. The feroryl groups link the entire structure to lignin.
Hemicellulose constitutes the second largest component of polysaccharides in perennial grasses, such as switchgrass and Miscanthus. Enzyme cocktails that hydrolyze hemicellulose into its major component sugars such as xylose (a 5-carbon sugar) and arabinose (a 5-carbon sugar) will significantly increase the fermentable sugars for biofuel production from lignocellulose-based feedstock. Enzymatic removal of hemicellulose by hemicellulases will also increase accessibility of cellulases to the cellulose component of plant cell walls or lignocellulosic feedstocks. Thus, the degradation of hemicellulose is a critical step in the utilization of lignocellulose feedstock for biofuel production.
Acid pretreatment is the current standard method for degrading hemicellulose prior to fermentation of its component sugars. This method, however, results in the production of toxic compounds that inhibit future fermentation. Thus, a significant need exists for improved pretreatment methods that can degrade hemicellulose without the production of toxic compounds.